Growth of the Plant
In a wheel that revolves evenly and without intermission it is difficult to find a starting point; every point is a beginning. The more carefully we look at the natural round the more we become conscious of the ceaseless character of its revolutions. Yet if a point must be chosen at which to make our first reflections, we cannot do better than choose the green leaf.
This wonderful mechanism is unique. The green leaf has the extraordinary capacity of securing the transformation of dead matter from the inorganic to the organic condition. It does this by changing simple mineral and chemical compounds into protoplasm which means living substance, so that we may say that if anything creates life on our planet it is the green leaf. Nothing else in Nature can do this. Were the green leaf to fail us, our civilization and culture would fade into nothingness, our existence would cease to have an origin, we ourselves should perish.
It operates by using the energy of sunlight. On this everything depends. The explanation of this process is very profound and to a large extent still hidden from us, but we may roughly say that the green leaf combines physics and chemistry in an act of creation, and that the created thing is organic matter in the shape of food -- food for the plant, food for the animal, and food for man. Every green leaf contains a substance called chlorophyll, which enables it to intercept the energy of light and perform this miracle of transmutation. Our own recent boasts about our extensive use of atomic energy are petty compared with this marvel, which has been going on for millions of years. There is no comparison, nor ever can be, between our puny strivings and the operations silently carried on by the plant world, operations massive, perpetual and universal. The work of the green leaf is the master process and the master secret of the world.
As is obvious, the leaf is part of a larger structure; we shall not be able to follow what the leaf does unless we know something of the whole organism, the plant. In plants is exemplified in a high degree that natural variety which we noted in our first chapter. The size of plants alone ranges from an alga so minute as to constitute, even when repeated a thousandfold, a mere green smudge on the surface of something else to a huge tree like an oak, which from its crown to its deepest root may measure two hundred feet and whose weight may run into many tons. Differences in shape, pattern, function and length of existence are no less; did we not know it we might be inclined to deny the identity of the daisy, the potato, the sugar cane, the apple tree, the vine, the fern, the elm, the coconut palm, to take at random a few examples of varied plant life. Yet we have no real difficulty in recognizing among all these variations a common obedience to the same laws. They are all rooted in the soil; they all seek air; they all grow; they all need the same food materials; this defines them as a single group of created things of like nature and function.
Their most obvious characteristic is their capacity for growth, for rapid growth, and in many cases for renewed growth. There is a great strength in this, surpassing what the animal can display, which unlike the plant does not easily survive great interference or injury and which certainly lacks the ability of so many plants to die down -half the year and spring up again more strongly than before with the return of the season. This persistent renewal of the plant is derived from its genius for finding and making its own food out of the most unpromising raw materials and sometimes despite the most unpromising circumstances. It is this food factory we propose to examine in the present chapter.
The plant draws the raw materials for its food from everywhere. It draws from the atmosphere; it draws from the soil, namely, from the gases in the soil, from the mineral particles therein, from the organic matter there, from certain forms of fungi (which are themselves also part of the plant world), it draws from water. A very few plants are piratical and feed on other plants and a few rare species feed direct on insects, but most plants are content with these three normal sources of food material, the air, the soil, the waters. Their mechanisms for searching out raw materials are by now highly developed and more than one alternative method of absorbing these materials has been acquired; plants are, so to say, insured against all risks. This is an important point.
On the whole, the sources of the food materials at the disposal of the plant are inexhaustible. Their availability is another matter. It often happens that the needed substances are there but that the plant is unable to use them. Either they are out of range or the plant is unable to exercise the capacity for taking them in. In cultivation by man this availability of the plant's requisites matters enormously.
The higher plants consist of two well-defined parts, one above and the other below ground. Both of these parts take in not food, but raw materials for making food; the real food always has to be manufactured by the plant itself for its own support; this is the great difference between plants and animals, for animals must find their food in a form which they can directly digest, even if, as in the case of ruminants, the digestion processes are prolonged. In order to have access to as much food material as possible the plant stem and branches above ground, the main and lateral roots below ground are spread out in such a way as to bring the leaves and the rootlets and root hairs, which are the agencies for gathering in the supplies, into contact with as much air space and soil space as possible; these fine instruments then comb their respective mediums for what they can get; there is not a square inch left unsearched. Moreover, each of these systems enters into further competition with the similar systems of other plants. Thus there is a great jostling for available food materials both in the air and in the soil. In the air every possibility of the sunlight is explored by Nature by the simple device of allowing different plants to attain different height levels and also by causing the general leaf canopy to develop not simultaneously but in successive stages; thus the mosses at the roots of the forest tree will be very bright green in the early spring, while they can still get the light before the higher tree foliage shades them-for that brief space it is their hour. In the same way the various layers of the soil are combed by the rival root systems at successive intervals and to successive depths. It is also known that plants withdraw slightly different constituents from the soil and that possibly they may help each other in some way to do this.
As far as possible the farmer must manipulate his crop to follow this intricate pattern in the plant's search for food materials. It is by no means always easy, and one of the risks of human cultivation is that too many of the same plant may be sown, growing simultaneously to an equal height and depth, thus leaving much air space and much soil space unused, and also drawing away too much of the same kind of raw material.
When completely nourished a plant is made up of water, which forms no less than 80 per cent of its structure and keeps it turgid and erect and thus able to reach the right level of height; of a number of complex organic substances; of a number of different mineral salts; and of proteins, which are in truth the most important of all. Of these constituents water is the only one which is absorbed by the plant, so to say, ready-made. All other materials, as we must repeat, have to undergo processes of manufacture within the plant.
The necessity of water for a plant needs no argument. It is obvious; without water no plant can grow. As we shall see presently, water is also the vehicle for conveying to the plant a large part of its food materials and for moving about the food when made. It might almost be said that the art of causing plants to grow is comprised in the art of giving them water in the right way; there is a great deal of skill in doing this. It has been argued that the Western nations in their cultivation are extravagant in their use of water, perhaps because it is so abundant. In the East water is used with more care and far more effectively.
One thing we find very hard to manage and that is the giving of water which is not merely sufficient in itself and rightly distributed, but which holds oxygen in solution, that is what rain consists of. As it comes down it washes out some of the oxygen in the air and dissolves it; it becomes highly impregnated. This makes it so extraordinarily refreshing. Now the roots of plants need oxygen as much as they need water, for oxygen gives the plant an essential factor for carrying on its manufacture of food. The rain is one important source from which the plant derives its oxygen.
The other is the atmosphere. That is why air, whether above or below ground, is the second great essential to plant growth. Plants must breathe, though, as we shall see presently, they do this in a way differing from that of animals. They need air, however, as much as any animal. Thus water and air are the two prerequisites of the growth of a plant; they condition the intake of all other raw materials. Even a partial deficiency in either causes plant life to sink to a low level. Lack of water produces the desert with its scanty bushes, lack of air produces the marsh with its depressed forms of vegetation.
We stated above that the green leaf has the faculty of absorbing its raw materials and causing them to combine within itself, and that these substances have to be simple or nearly simple elements. This at once introduces a limitation. Contrary to what we might at first suppose, the plant cannot feed immediately off anything in the soil. The soil has to do its part also. It is impossible to understand the growth of the plant unless we have some idea of the life of the soil. The plant cannot, for instance, absorb inert minerals except by a sucking action -- osmosis -- they have therefore to be dissolved. Nor can it find its food direct either from decaying matter or from animal excreta; it has to wait until these substances have been prepared for it. In fact, the plant world which supplies us and all other animals with abundant food to eat is, in this respect, dependent on yet a third group of created things to supply its own needs. This group is the soil flora, the great soil proletariat, as it has been called. The soil flora, as its name implies, is itself in some ways like a growth of plants; in other ways it more resembles a swarm of tiny insects, but its components are so ultra-minute -- they are described in science as microflora -- that it is rather futile to institute such decriptions. The soil flora has one thing, however, in common with the plant world; it is exceedingly varied. This applies both to the soil fungi, which are really themselves forms of plants, mostly very minute, and also to the soil bacteria, which are quite invisible to the naked eye. A number of families, genera and species of both fungi and bacteria have been classified and examined; they have different functions to perform, sometimes successive to each other, but often in competition. This soil flora, it will be realized, is alive.
In breaking down the organic matter in the soil the fungi act first. They can be seen as tiny threads of grey matter clinging to dead roots and twigs; they attack this mineral and thrive and multiply on what they find. The bacteria succeed them and complete the process as the second agents. They incorporate into themselves much of what they eat and form microbial protein (the fungi also have a high protein content); at the same time they manufacture as an end product the inert mineral salts, largely nitrates with some phosphates, potash and so forth, which form a portion of the food materials of plants. It is at this point that the Wheel of Life revolves at its lowest; it is here that what was once living is being reduced to what is inert and inorganic. The soil flora are the selected agents for this devolution, which only the plant can reverse.
When the microflora have done their work there is a most varied assortment of substances in the soil. There are the original soil elements -- these occur as constituents of a variety of minerals in conformity with the general composition of the underlying rocks and of the subsoil from which the topsoil was originally derived; there are quantities of undecayed organic waste; there are the mineral salts to which the fungi and bacteria have finally reduced the remainder of these wastes, and these exist in every stage of their descent to the inorganic phase; there are the fungi and bacteria themselves, active, and at work; and there are their bodies when dead, amalgamated with the undecayed portions of the vegetable and animal wastes in the form of humus.
This mass is not without a system. The soil water is the great binding agent which unites it all. The soil water is the habitat of the soil microflora which actually live in it. It takes the form of a thin film of moisture bathing each soil particle. The water which makes it up is derived both from above and from below; from above by the percolation of the rain, from below by the rise of the subsoil water through capillary action. The soil water is in constant slow movement; it is also impregnated with that dissolved oxygen which we noted as washed down by the rain out of the atmosphere and this oxygen is drawn on by the soil flora when they manufacture their nitrates and other compounds. The supply of oxygen is therefore as important to the soil flora as it is to the plant. If the soil solution is fully oxygenated and moving, conditions are favourable to the life of the soil; if it is deprived of oxygen and stagnant, there is a great slowing down in the activity of the soil inhabitants.
'The soil solution, however, is not simply oxygenated water. Besides oxygen it also holds dissolved carbon dioxide. This is derived in part from the air but mostly from the fermentation of organic wastes, a process which always produces this gas. By absorbing carbon dioxide the soil solution becomes slightly acid, and it is this mildly acid character which enables it to perform the important function of the dissolving of minerals. Both the original mineral elements in the soil, and also the compounds which the fungi and bacteria have produced out of the organic wastes, are then easily held dissolved in the soil solution.
It will be seen from this account that the state of the soil solution, as regards its impregnation both with organic compounds and also with minerals, depends on there being organic matter giving off carbon dioxide in the soil. It is particularly important to note that the soil solution is unlikely to be richly impregnated with minerals unless it is also in contact with an abundant supply of decaying organic matter. It has not the power to dissolve the minerals unless it can attain the necessary slight degree of acidity given by the continuous absorption of carbon dioxide. Any idea that the soil solution can be mineral-rich without being also organically supplied to an ample degree is futile.
It is the soil solution which is the vehicle for transferring the food materials in the soil to the plant. It is sucked in by the root hairs (osmosis), entering through the cellulose walls, carrying both water and supplies of the materials which the plant needs to manufacture its own food. This is the straightforward and obvious way in which plants derive their raw materials from the soil.
But there is a good deal more to be considered. The soil solution does not by any means satisfy the needs of the plants. It has to draw on other resources, more especially on the atmosphere; it has also to draw on the soil itself in rather more special ways. While the soil solution gives it part of its food materials, namely, organic compounds and inert salts, it also needs large supplies of carbon and some extra nitrogen. In fact, the range of food materials which the plant requires is surprising; it needs most of the important elements and many of the rarer ones. Into this wider question we cannot enter, and shall do well to conclude by concentrating briefly on its need for the two elements just named, carbon and nitrogen.
Carbon is one of the first requirements of the plant as it is of all living things, forming, as it does, the core of the material structure of all that we see around us. To obtain carbon the leaves of a plant breathe in carbon dioxide gas from the atmosphere; the chlorophyll of the plant, using the energy of light during the day, builds up this gas into the food on which the plant lives -- carbohydrates, proteins and so forth. These carbohydrates and proteins not only feed the plant but they have a subsequent career of their own; they are the only source of food on this planet, they feed the animals and ourselves. The atmospheric supply of the carbon dioxide needed by the plant is a product of the respiration of animals (and also in part of plants themselves); animals breathe in the oxygen of the air, use it to burn up slowly the food in their own bodies and then exhale this carbon dioxide. The green leaves also give out oxygen, the source of which is still undetermined. The old idea was that this came from the carbon dioxide of the air, but recently it has been suggested that the source is water. The processes of the plant and animal are thus reversed during a large part of the twenty-four hours.
This means that there is a perpetual circulation of carbon dioxide, ensuing the circulation of carbon, in and out of the atmosphere, the plant balancing what the animal does, or, if we like to say so, purifying the atmosphere vitiated by the animal. Were this not so, were plants not there to remove the carbon dioxide, this gas would accumulate in the air and we could no longer keep alive. This is the first great interlocking act of the plant and animal world in Nature, the first point at which these two groups fit into and supplement each other's functions.
Although the atmosphere only contains 4 parts of carbon dioxide by volume in every 10,000, this is sufficient; there is no carbon problem for plants. This means that there is no carbon problem for any of the other living things in Nature. We must never forget that it is the plant and sunlight which secure the carbon needed out of this inexhaustible atmospheric supply, and which start it on its career in the organic phase, thus enabling the structures and bodies of all created things to build themselves up into solid forms.
The problem of nitrogen is quite different. It is curious that this singularly inert gas should be the basis of the life-principle. There is a surpassing need for it in all living things. Nitrogen is required to form the proteins, and these, as we shall constantly have to emphasize, are the substances which hold the secret of growth, health, well-being and reproductive power. It is not surprising, therefore, that every source of nitrogen which can be found should be called upon. There are three sources: first the nitrogen existing as gas in the atmosphere, which is known as uncombined nitrogen or free nitrogen; second, the nitrogen existing as organic compounds either in decaying matter in the soil or in the dropped excreta of animals; and, third, the nitrogen existing as inorganic compounds resulting from the final breakdown of these two materials; both the last forms of nitrogen are usually referred to as combined nitrogen because they exist in combination with other elements.
The plant has some difficulty in assimilating any of this nitrogen. As we have seen it has to wait for the soil flora to break down the organic wastes, and then has to wait further for the soil solution to dissolve the products thus produced; only when all this has been done can it get at these food materials. It is therefore scarcely surprising to find that plants can be starved of combined nitrogen. This shows itself in a stunted condition, in paleness of colour, in general weakness, and finally in failure of reproductive power. It is also possible by human agency to supply plants with too much nitrogen, which betrays itself in a superabundance of rank foliage and very dark unnatural colour, but perhaps this is a question which need hardly be considered at this point. That plants in a natural condition have something of a struggle for nitrogen may be admitted, and it is interesting to follow the special mechanisms which some of them have evolved to obtain extra amounts. What is present in the soil is an item for competition between plant and plant, so that any extra supplies to be obtained have to be looked for from the inexhaustible volumes present in the atmosphere.
Can plants absorb something from these inexhaustible volumes? If they could, their life would be easy, for the supply is super-abundant, namely, 75 per cent of the total volume of the atmosphere. But only a few plants, and algae and some other low minute forms of vegetation like lichens, can absorb the free atmospheric nitrogen-this process is known as nitrogen fixation. Nevertheless, though exceptional, the capacity which they exercise is important. When such plants die, the nitrogen they have collected in the organic condition is added to the nitrogen reserve in the soil, and this can be a material help to a succeeding crop. This process is not cumulative; the extra nitrogen may be lost again by a process of denitrification. But for the time being the collection made by these forms of vegetation can constitute an important addition to available supplies.
Exactly the same thing is done by a certain group of soil bacteria; these are known as Azotobacter. The Azotobacter also can fix nitrogen from the air and add it to the soil. These bacteria are most active in well-aerated fertile soil containing a sufficiency of carbonate of lime, which means that a soil already well supplied with what the plant wants will be able to find those extra sources of combined nitrogen, whereas a soil already infertile will not be able to get at them. The plant can profit by the result. It can also evolve a mechanism of its own, which, however, applies only to a certain group of plants, the legumes. These have acquired the faculty of growing on their roots, sometimes also on their stems, nodules, which look like tiny balls or buttons visible to the eye; these nodules are homes for colonies of Bacillus radicicola, in which there collect the free nitrogen of the air and store it in the combined form. Such plants come to carry banks of extra nitrogen on themselves, and again these reserves can help other crops. In wild life a certain number of such leguminous plants like the wild white clover are always in existence and add to the richness of the earth's green carpet; in our own cultivation processes it has become customary to include such a crop in our rotations for the purpose of getting the advantage of the extra nitrogen; this is added to the soil by means of digging in the haulms of the nodule-bearing plant, for instance, peas or beans. (This practice is discussed in Chapter 6.)
There is one other way in which plants absorb extra nitrogen, and it is quite a distinct one, for the nitrogen taken in is already in the form of living protein. Thus the rule by which the plant undertakes the manufacture of its own food is abandoned, and most exceptionally it consents to feed off something which has been made for it. This substance is a fungus or, to use the scientific term, a mycelium, which starts as an independent web-like growth, visible to the eye, surrounding the sheath of the plant rootlet, goes on to penetrate the walls of the rootlet which it has first surrounded and then entered. The mycelium has a very high protein content, i.e. a very high nitrogen content.
This curious process, which is known as a "mycorrhizal association" from the two Greek words for fungus and root, is common to a great number of plants; where it does riot occur it would appear to be replaced by parallel but even more intricate arrangements for the intake of proteins. It ranks as a symbiosis, i.e. as a living together, a true partnership, for the fungus and the plant eventually terminating the arrangement. We shall have occasion to discuss its full bearing later. Here it is enough to rank it as a significant method employed by the plant to get at a supply of extra combined nitrogen, and what is more at a very special form of nitrogen -- a method which is unique in itself and again one additional to the ordinary processes used by the plant to obtain this element.
The co-existence of these different ways of getting at nitrogen tell their own story; they show the plant's need. If plants may be said to jostle and fight each other for sunlight and air and water in a rivalry which can be deadly, they are equally_ pertinacious in competing without mercy for the available supplies of nitrogen.
It will have been realized by now that the growth of plants is a highly organized affair. Their ways of getting raw food materials are varied; they are at once greedy and fastidious collectors. Thus their relationships with their environment come to be intricate; they have one set of contacts with the atmosphere -- and these in themselves are not simple -- other contacts with the soil solution and the supply of water, and yet other contacts with the soil and all it contains. In three different directions they are at work, having to change their performance continuously, rapidly, and effectively with alterations in light, fluctuations in temperature, and according to the presence or absence of moisture. Perhaps the point on which most stress needs to be laid, partly because at first it might escape notice, is the relation of plants to the soil microflora. This relation is truly dynamic. Both plants and soil microflora are alive; both therefore need certain elements, oxygen, carbon, nitrogen. The passing to and fro of the last named is. extraordinarily complicated -- we have barely indicated a small part of what goes on. It is easiest to say, once for all, that plants and microflora, operating as they do in the same medium and simultaneously, mutually dependent on each other's results and yet capable of being in rivalry, are like two cogwheels in the same system which are closely geared together, the slightest movement in the one inevitably setting up a consequential movement in the other. The life of the soil and the plant are a series of dramatic sequences of which we see the result in the growth of the green leaf.
Of the importance of the function of that green leaf there can be no doubt; on the continuance of the earth's green carpet all that we are depends.
Next: 3. The Agricultural Effort and its Reward
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